1. Field of the Invention
The present invention relates to an AM stereo receiver for receiving AM stereo broadcasting.
2. Description of the Prior Art
An AM stereo receiver receives an RF AM stereo signal of the AM stereo broadcasting system. The AM stereo broadcasting is now under study for practical operation in near future.
One of RF AM stereo signals of AM stereo broadcasting systems which are now regarded as very promising is an RF AM stereo signal of an AM-PM system (such as a Magnavox system) and another is an RF AM stereo signal of an AM-FM system (such as a Belar system).
The RF AM stereo signal of the AM-PM system is one that a radio-frequency carrier (in an AM broadcasting frequency band ranging from 535 to 1605 KHz both in U.S.A. and Japan) phase-modulated by a difference signal (L-R) has been amplitude modulated by a sum signal (L+R). The difference signal (L-R) is a difference signal between a left signal (L) having an audio frequency and a right signal (R) having an audio frequency. The sum signal (L+R) is a sum signal of the right and left signals (R) and (L).
Th RF AM stereo signal of the AM-FM stereo system is one that a radio-frequency carrier frequency-modulated by the difference signal (L-R) has been amplitude modulated by the sum signal (L+R).
A conventional AM stereo receiver comprises an IF AM tuner for selecting an RF AM stereo signal and converting it into an IF AM stereo signal the carrier of which has a lower intermediate frequency (such as 450 KHz) than the carrier frequency of the RF AM stereo signal; an IF AM signal amplifier for amplifying the IF AM stereo signal from the IF AM tuner; a limiter for amplitude limiting the amplified IF AM stereo signal to obtain an IF PM (or FM) difference signal is one that a carrier of a frequency equal to the frequency of the non-phase- (or frequency-) modulated carrier of the amplified IF AM stereo signal has been phase (or frequency) modulated by the difference signal (L-R); a PM (or FM) demodulator for phase (or frequency) demodulating the IF PM (or FM) difference signal from the limiter into the difference signal (L-R); an AM demodulator for amplitude demodulating the IF AM stereo signal from the IF AM signal amplifier into the sum signal (L+R); and a matrix circuit for deriving the right and left signals (R) and (L) from the difference signal (L-R) from the PM (or FM) demodulated and the sum signal (L+R) from the AM demodulator.
With the AM stereo receiver of such a construction, the IF AM stereo signal available from the IF AM signal amplifier is such a signal that a carrier having an intermediate frequency has been amplitude modulated by the sum signal (L+R). If the IF PM (or FM) difference signal from the limiter has not sufficiently been amplitude limited, then the difference signal (L-R) from the PM (or FM) demodulator contains the sum signal (L+R) component, with the result that the right and left signals (R) and (L) available from the matrix circuit cannot well be separated from each other. To avoid this, the the limiter must be arranged to sufficiently amplitude limit the IF AM stereo signal so that the resulting IF PM (or FM) difference signal may be free from amplitude variations.
In the case where the limiter has such a construction, however, a spurious signal composed of a fundamental wave component of a frequency equal to the intermediate frequency (hereinafter referred to as 450 KHz for the sake of brevity) of the IF AM stereo signal and its higher harmonic components radiate from the limiter at relatively high levels and this spurious signal centers into the RF AM tuner, the IF AM signal amplifier and so forth to adversely effect them. The frequency of the fundamental wave component of the spurious signal is 450 KHz and the frequencies of second, third, fourth, fifth . . . harmonic components are 900, 1350, 1800, 2250 KHz . . . On the other hand, the RF AM tuner handles an RF AM stereo signal having a carrier frequency of 535 to 1605 KHz, a local oscillation signal having a frequency in the range of 985 (=535+450) to 2055 (=1605+450) KHz or 85 (=535-450) to 1155 (1605-450) KHz and an IF AM stereo signal having a carrier frequency of 450 KHz. Accordingly, the frequencies of the fundamental wave component and the second, third and fourth harmonic components are in the frequency range of the signals handled by the RF AM tuner. As a result of this, the operation of the RF AM stereo tuner is disturbed by the fundamental wave component and the second, third and fourth harmonic components, so that no good reception is obtained. To avoid such disturbance, it is necessary to take a step such as, for example, electromagnetic shielding of the limiter and this makes the AM stereo receiver bulky, complex and expensive as a whole.